Use ContentControl in DropDown template.

Instead of `ContentPresenter`. This fixes a memory leak whereby
`DropDown` wasn't correctly parenting the `Rectangle` created for
`SelectionBoxItem` when the selected item is a control. Fixes #706.

src/Avalonia.Themes.Default/DropDown.xaml

@@ -10,10 +10,10 @@
                 BorderBrush="{TemplateBinding BorderBrush}"
                 BorderThickness="{TemplateBinding BorderThickness}">
           <Grid ColumnDefinitions="*,Auto">
-            <ContentPresenter Content="{TemplateBinding SelectionBoxItem}"
+            <ContentControl Content="{TemplateBinding SelectionBoxItem}"
-                              Margin="{TemplateBinding Padding}"
+                            Margin="{TemplateBinding Padding}"
-                              HorizontalAlignment="Left"
+                            HorizontalAlignment="Left"
-                              VerticalAlignment="Center"/>
+                            VerticalAlignment="Center"/>
             <ToggleButton Name="toggle"
                           BorderThickness="0"
                           Background="Transparent"

tests/Avalonia.Controls.UnitTests/DropDownTests.cs

@@ -1,20 +1,65 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
-using System;
 using System.Linq;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
+using Avalonia.Controls.Shapes;
 using Avalonia.Controls.Templates;
-using Avalonia.Platform;
+using Avalonia.LogicalTree;
-using Ploeh.AutoFixture;
+using Avalonia.Media;
-using Ploeh.AutoFixture.AutoMoq;
+using Avalonia.UnitTests;
+using Avalonia.VisualTree;
 using Xunit;
 
 namespace Avalonia.Controls.UnitTests
 {
     public class DropDownTests
     {
+        [Fact]
+        public void SelectionBoxItem_Is_Rectangle_With_VisualBrush_When_Selection_Is_Control()
+        {
+            var items = new[] { new Canvas() };
+            var target = new DropDown
+            {
+                Items = items,
+                SelectedIndex = 0,
+            };
+
+            var rectangle = target.GetValue(DropDown.SelectionBoxItemProperty) as Rectangle;
+            Assert.NotNull(rectangle);
+
+            var brush = rectangle.Fill as VisualBrush;
+            Assert.NotNull(brush);
+            Assert.Same(items[0], brush.Visual);
+        }
+
+        [Fact]
+        public void SelectionBoxItem_Rectangle_Is_Removed_From_Logical_Tree()
+        {
+            var target = new DropDown
+            {
+                Items = new[] { new Canvas() },
+                SelectedIndex = 0,
+                Template = GetTemplate(),
+            };
+
+            var root = new TestRoot { Child = target };
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            var rectangle = target.GetValue(DropDown.SelectionBoxItemProperty) as Rectangle;
+            Assert.True(((ILogical)target).IsAttachedToLogicalTree);
+            Assert.True(((ILogical)rectangle).IsAttachedToLogicalTree);
+
+            rectangle.DetachedFromLogicalTree += (s, e) => { };
+
+            root.Child = null;
+
+            Assert.False(((ILogical)target).IsAttachedToLogicalTree);
+            Assert.False(((ILogical)rectangle).IsAttachedToLogicalTree);
+        }
+
         private FuncControlTemplate GetTemplate()
         {
             return new FuncControlTemplate<DropDown>(parent =>
@@ -26,8 +71,7 @@ namespace Avalonia.Controls.UnitTests
                     {
                         new ContentControl
                         {
-                            Name = "contentControl",
+                            [!ContentControl.ContentProperty] = parent[!DropDown.SelectionBoxItemProperty],
-                            [~ContentPresenter.ContentProperty] = parent[~DropDown.SelectionBoxItemProperty],
                         },
                         new ToggleButton
                         {
@@ -35,7 +79,12 @@ namespace Avalonia.Controls.UnitTests
                         },
                         new Popup
                         {
-                            Name = "popup",
+                            Name = "PART_Popup",
+                            Child = new ItemsPresenter
+                            {
+                                Name = "PART_ItemsPresenter",
+                                [!ItemsPresenter.ItemsProperty] = parent[!DropDown.ItemsProperty],
+                            }
                         }
                     }
                 };